Recent development of NIR optical probes for molecular imaging holds great promise for breast cancer imaging. Targeted and activable probes interact with specific targets and therefore make it possible to image biological processes at the cellular or molecular level. Targeted probes bind to specific targets, such as receptors, and can be used to image receptor expression and distribution, while activable probes can be employed to image the activity of matrix-degrading enzymes such as MMPs or cathepsins. Such probes along with optical imaging may yield a unique, highly sensitive technology for in vivo imaging of tumor formation and metastasis. It is inevitable that in the near future some of these probes will be available for use in clinical settings. Despite the advances in molecular imaging technology, however, only recently efforts have been focused on developing in vivo fluorescent enhanced optical tomography (FDOT) that can be used in clinical settings for breast cancer imaging. In this application, we propose to employ a multi-modality approach, a combined MR/FDOT animal imaging system, to improve the accuracy of FDOT and in turn to improve the performance of MRI in distinguishing benign and malignant breast cancers. Several studies have proven that MRI detects malignant cancers which are occult on mammogram and ultrasound, and as such it has fast becoming the most popular imaging modality for screening young women. However, despite its high sensitivity, MRI has low specificity, such that it also detects many benign lesions. As a counterpart of this combined system, FDOT will increase the specificity of the MRI substantially using the targeted/activated probes. Similarly, MRI will improve the quantitative accuracy of FDOT by providing a-priori information, which makes the proposed system a true multi-modality imaging approach. Once the combined system is developed and optimized with phantom studies, the performance of the system will be evaluated with animal studies by using MMP sensitive activable fluorescence imaging probes and two different cell lines that express different levels of MMP activity. The proof-of-principle hybrid MR/FDOT imaging modality proposed in this application is an innovative approach and a first step towards such a clinical multi-modality imaging system. With appropriate modifications in design, the proposed technology has a great translational potential for future human clinical trials. PUBLIC HEALTH RELEVANCE Although mammography is very sensitive in detecting early breast cancer, it does not work well in women who have dense breast tissues, breast implants, or scar tissues due to previous treatment. Breast MRI is considered as the optimal modality in these patient populations. However, the currently available clinical MR contrast agent Gd-DTPA is a low molecular weight extracellular agent that does not provide high specificity in breast cancer characterization. In the mean time, the recent developments in the molecular imaging probes showed that it is very possible to have some of these exciting probes for clinical use in the near future. Therefore, we focused our efforts on to improve the specificity using a fluorescence tomography system as adjunct to MRI along with targeted/activable probes to increase the specificity. These probes are now only available for animal imaging and it is necessary to evaluate the performance of such a system with animal studies. Hence, this application is aimed at developing a hybrid MR/FDOT system for imaging of rats. If successful, the instrumentation developed during this project can be easily modified for clinical breast imaging and the outcome of our effort will offer a sophisticated tool that could provide high sensitivity and high specificity in the detection and characterization of tumors.